The miniaturization of semiconductor devices such as DRAMs and MRAMs proceeds more and more. In order to advance fine processing in the fabrication of such semiconductor devices, it is very important to make ions enter vertically the surface or a concave part to be processed. For example, as shown in FIG. 5 of Patent Document 1, in burying a hole having a high aspect ratio by sputtering, how to make the sputtering particles enter vertically relative to the hole bottom affects largely burying characteristics of the hole. As shown in FIG. 5 of Patent Document 1, the more sputtering particles are made to enter vertically the hole bottom, the more burying characteristics are improved.
As a method for realizing such a state, there is a so-called ion sputtering method, in which sputtering particles are ionized by a certain method and made to enter vertically relative to a substrate holder on which the substrate is placed (see Patent Document 2). In order to make ions enter vertically the substrate, DC bias or RF power negative for the substrate is applied to the substrate holder to generate a negative voltage for the substrate, thereby pulling ions in the substrate holder. In doing so, when the voltage of 100 to several hundreds volt is generated, since thermal energy of sputtering particles is considered to be about 0.1 eV at most, if no collision of ions occurs between a target and the substrate holder, ions enter the substrate substantially vertically.
However, actually, ions are scattered due to collision with another particle occurring between the target and the substrate holder to result in a broadened incidence angle.
Accordingly, in order to make ions enter the substrate vertically, it is essential to allow ions to reach the substrate without the collision with another particle in an accelerated region.
Here, the region in which ions are accelerated is a region called a cathode fall or a sheath existing near a cathode. Much of potential variations being generated during the discharge are generated in this part. The thickness thereof depends on the pressure in the discharge space, the power applied to the substrate holder, etc., and, typically, is about from 10 to 30 mm.
Accordingly, if ions are allowed to pass the region of the cathode fall or sheath without collision, it becomes possible to make an ion beam having a small divergence angle enter the substrate.
Here, an average distance in which a particle can move without collision is called a mean free path.
If the mean free path in an actual process gas is known, by adjusting the process condition to make the mean free path longer than the length of the cathode fall or sheath, incidence ions having a small divergence angle can be obtained.
Moreover, in dry etching, too, the same is applicable. In order to perform etching leading to a highly anisotropic shape, it is necessary to make ions of a small divergence angle enter a surface to be etched in a beam-like shape. In this case, too, if the mean free path in an actual etching gas is known, by adjusting the process conditions to make the mean free path longer than the length of the cathode fall or sheath, it becomes possible to make ion beams of a small divergence angle enter the surface to be etched.    [Patent Document 1] Japanese Patent Laid-Open No. 8-203827    [Patent Document 2] Japanese Patent Laid-Open No. 2001-35919    [Patent Document 3] Japanese Patent Laid-Open No. 2001-165907    [Non-patent Document 1] John F. O'Hanlon, “Vacuum Technology Manual”, SANGYO TOSYO, PP 7 to 10    [Non-patent Document 2] Hiroo Kumagai, three others, “Vacuum Science and Engineering”, Shokabo, pp 43 to 49